This invention relates to sulfur silane coupling agents containing multiple blocked mercapto groups which are latent, that is, they are in a state of reduced activity until such a time as one finds it useful to activate them. The invention also relates to the manufacture of mineral filled elastomers, rubbers and inorganic fillers comprising these silane coupling agents, as well as to the manufacture of the silanes.
The majority of art dealing with of sulfur-containing coupling agents in mineral filled elastomers involves silanes containing one or more of the following chemical bond types:
S—H (mercapto), S—S (disulfide or polysulfide), C═S (thiocarbonyl) or C(═O)S (thioester). Mercaptosilanes have high chemical reactivity with organic polymers used in mineral filled elastomers and therefore effect coupling at substantially reduced loadings. However, these chemical bonds between the coupling agent and the organic polymer are weaker than the carbon-carbon bonds of the organic polymer. Under high stress and/or high frequency use conditions, these chemical bonds are susceptible to breakage and, therefore, loss of coupling between the organic polymer and the coupling agent. The loss of coupling may contribute to the wear and to the degradation of other elastomeric physical properties. The high chemical reactivity of mercaptosilane coupling agents with organic polymers also leads to unacceptably high viscosities during processing and premature curing (scorch). Their undesirability is aggravated by their odor. As a result, other, less reactive coupling agents such as the coupling agents that contain the S—S (disulfide and polysulfide), C═S (thiocarbonyl) or C(═O)S (thioester) functional groups are used. Because these silane coupling agents are less reactive with the organic polymers, they require higher use levels and often do not achieve the same level of bonding. Similar to the mercaptosilane coupling agents, these sulfur silanes are bonded to the organic polymer through a C—S bond.
The prior art discloses acylthioalkyl silanes, such as CH3C(═O)S(CH2)1-3Si(OR)3 (M. G. Voronkov et al. in Inst. Org. Khim., Irkutsk, Russia) and HOC(═O)CH2CH2C(═O)S(CH2)3Si(OC2H5)3 (U.S. Pat. No. 3,922,436 to R. Bell et al.). Takeshita and Sugawara disclosed in Japanese Patent JP 63270751 A2 the use of compounds represented by the general formula CH2═C(CH3)C(═O)S(CH2)1-6Si(OCH3)3 in tire tread compositions; but these compounds are not desirable because the unsaturation α,β to the carbonyl group of the thioester has the undesirable potential to polymerize during the compounding process or during storage. Prior art by Yves Bomal and Olivier Durel in Australian Patent AU-A-10082/97 discloses the use in rubber of silanes of the structure represented by R1nX3-nSi-(Alk)m(Ar)p—S(C═O)—R (Formula 1P) where R1 is phenyl or alkyl; X is halogen, alkoxy, cycloalkoxy, acyloxy, or OH; Alk is alkyl; Ar is aryl; R is alkyl, alkenyl, or aryl; n is 0 to 2; and m and p are each 0 or 1, but not both zero. This prior art, however, stipulates that compositions of the structures of Formula (1P) must be used in conjunction with functionalized siloxanes. The prior art does not disclose or suggest the use of compounds of Formula (1P) as latent mercaptosilane coupling agents, nor does it disclose or suggest the use of these compounds in any way that would give rise to the advantages of using them as a source of latent mercaptosilane. In addition, these patents do not describe coupling agent that have multiple thioester groups in the appropriate stereochemical configuration to foster multiple linkages to the organic polymer.
U.S. Pat. Nos. 6,608,125; 6,683,135; 620,439; 6,127,468; 6,777,569; 6,528,673 and 6,649,684, US Patent Publication Nos. US20050009955A1, 20040220307A1, 2003020900A1, 20030130388A1, and application Ser. Nos. 11/105,916 and 10/128,804, and European patent application EP1270657A1 teach the use of blocked mercaptosilanes of the structure represented by [[(ROC(═O))p-(G)j]k-Y—S]r-G-(SiX3)s, where Y is a polyvalent blocking group (Q)zA(=E) and r is an integer 1 to 3 in rubber compounds and s is preferably 1 to 3, in rubber master batches and as a surface treatment for mineral fillers and how to manufacture the silane. Although these patents and patent applications disclose structures that possess more than one blocked mercapto group, i.e. r=2 or 3, they do not teach the specific stereochemical configurations of the polyvalent G structure between the silicon atom and the organofunctional group necessary to achieve the efficient multiple bonding between the coupling agent and the organic polymer.
U.S. Pat. No. 4,519,430 to Ahmad et al. and U.S. Pat. No. 4,184,998 to Shippy et al. disclose the blocking of a mercaptosilane with an isocyanate to form a solid which is added to a tire composition, which mercaptan reacts into the tire during heating, which could happen at any time during processing since this is a thermal mechanism. The purpose of this silane is to avoid the sulfur smell of the mercaptosilane, not to improve the processing of the tire. Moreover, the isocyanate used has toxicity issues when used to make the silane and when released during rubber processing.
U.S. Pat. No. 3,957,718 to Porchet et al. discloses compositions containing silica, phenoplasts or aminoplasts, and silanes, such as xanthates, thioxanthates, and dithiocarbamates; however, the prior art does not disclose or suggest the use of these silanes as latent mercaptosilane coupling agents, nor does it suggest or disclose the advantage of using them as a source of latent mercaptosilane.
U.S. Pat. Nos. 6,359,046; 5,663,226; 5,780,531; 5,827,912; 5,977,225; 4,709,065; 6,759,545 and WO 2004000930A1 disclose a class of polysulfide silane coupling agents that contain more than one S—S (disulfide or polysulfide) functional groups per molecule. However, the multiple S—S linkages are achieved by separating the functional groups with an organic hydrocarbon radical. In use, these S—S groups decompose to form sulfur radicals that couple to the polymer, but generate species that contain only one sulfur reactive group per silicon atom. Dittrich, et al. in U.S. Pat. Nos. 5,110,969 and 6,268,421 and Weller, et al., overcame this feature. They disclosed structures that contain more than one sulfur functional group directly attached to silicon atom through a cyclic hydrocarbon radical. The multiple S—S groups were bonded to adjacent carbon atoms and the silicon atoms were directly attached to the rings through hydrosilation of the alkoxysilane to a vinyl containing cyclic hydrocarbons. However, these compounds contained rings of S—S and carbon atoms or were polymeric materials wherein the silyl containing hydrocarbon radicals were connected through S—S groups. These cyclic or polymeric coupling agents were rendered less reactive with the organic polymers because they contained S—S groups attached directly to secondary carbons. The attachment of the S—S containing group to secondary carbon atoms sterically hinder the reaction of the S—S groups and inhibit their reactions with the organic polymers.
Therefore, a need exists for latent coupling agents that have low reactivity to affect processing of the mineral filled elastomers or rubbers without scorch and can be activated at the desired time to form multiple linkages with the organic polymer. These multiple linkages provide sufficient bonding so that the loss of coupling between the rubber and coupling agent is minimized during high stress or frequency use conditions, such as is experienced by tires, without exhibiting the disadvantages such as described herein.